1. Field of the Invention
The present invention relates to a magnetic tape cartridge, in which a single reel, a magnetic tape is wound thereon, is rotatably stored, and is composed of an upper half, a lower half, an opening for drawing the magnetic tape, and a slide door for achieving the opening-and-shutting of the opening. More specifically, the present invention relates to a spring member, which impels a slide door to the shutting direction for shutting the opening of the magnetic tape cartridge.
2. Description of the Related Art
In the magnetic tape cartridge used as the external recording medium for recording the back up data of a computer etc, a single reel, a magnetic tape is to be wound, is rotatably stored within a cartridge case. This cartridge case is composed of an upper half and a lower half, and has a nearly flat square shape.
In the occasion of using, a magnetic tape is drawn out from the cartridge case, and is wound by the reel mounted on a read-write apparatus. Thus the magnetic tape is traveled.
An opening for drawing the magnetic tape is formed on the sidewall of the cartridge case. This opening is normally covered for preventing the intrusion of the dust, and is opened at the time of drawing a magnetic tape for recording or replaying the data on the magnetic tape using the recording-writing apparatus.
In the magnetic tape cartridge as represented by LTO (Linear tape open circuit) standard among these conventional magnetic tape cartridges, a slide door for achieving the opening-and-shutting of the opening is arranged so as to slide along the inside of the sidewall of the cartridge case.
The slide door is fit with a guide groove formed on the upper half and the lower half, respectively, for smoothly achieving the opening-and-shutting movement along the guide grooves. The slide door is impelled to the shutting direction by the spring member except for at the time of using.
As an construction of such type of magnetic tape cartridge, as shown in FIG. 17, an guide groove 101b is provided on the lower half 101 along the side wall 101a. A hooking part 101d having a canaliform shape is formed on a rib 101c provided at the inward side of the side wall 101a. 
In this construction, a spring arm 102a of the spring member 102 is hooked on the hooking part 101d, and a spring arm 102b is fit on a notching part 103a provided in the opening direction side of the slide door 103. Thus, the slide door 103 is impelled toward the shutting direction side.
For achieving the certain hooking with the hooking part 101d, a tip part 102c of the spring arm 102a is bent in the perpendicular direction with respect to the rib 101c. 
On the other hand, for achieving the certain contact with the notching part 103a, the spring arm 102b is bent toward the slide door 103 side, and also for achieving the certain hooking and impelling of the notching part 103a, a tip part 102d of the spring arm 102b is bent in the shutting direction side.
When assembling the magnetic tape cartridge having a conventional construction, after attaching the spring arm 102a to the hooking part 101d, the slide door 103 is attached to the lower half 101.
At this time, as shown in FIG. 18A, the slide door 103 is pushed toward the opening direction side while keeping the condition that the opening direction side thereof is tilting downwardly, and then engaged to the guide groove 101b. 
In this assembling manner, as shown in FIG. 18B, since the bottom end of the slide door 103 is stuck with the tip part 102d of the spring arm 102b, the opening direction side of the slide door 103 should be picked up for releasing from the stuck condition and for achieving the assembling.
In the assembling of the spring member and the slide door, consequently, thousands of man-hours are required, and the automatization of the product line of a magnetic tape cartridge had been made difficult.
In the assembling of the magnetic tape cartridge having a structure of shown in FIG. 19, the magnetic tape cartridge is assembled as below.
First, each parts, such as a reel, a breakable tab for preventing the accidentally erase, and the likes, are disposed at the predetermined position of the lower half. And then, as shown in FIG. 19, a spring member 201 and a slide door 202 are placed on the guide groove 203a of the lower half 203. Hereat, the spring member 201 is arranged in the bridging condition so that the slide door 202 is constantly impelled to the shutting direction side.
Until the upper half (not shown) and the lower half 203 are assembled together, however, only the lower part of the slide door 202 is supported by an slider stop 204 of the lower half 203. To be more precise, about ⅕ of an overall height length of the slide door 202 is supported. In this state, since the upper part of the slide door 202 is not restricted, the slide door 202 tends to be lifted upwardly. This lifting movement is caused by the urging force brought by the spring member 201 which makes the slide door 202 rotate centering on the bottom end in the shutting direction side of the slide door 202.
When the assembling of the upper half and the lower half 203 is continued as it is, therefore, the engagement of the slide door 202 with the guide groove of the upper half or the lower half 203 is disturbed. As a result of this disturbance, the automatization of the product line of the magnetic tape cartridge becomes difficult.